Biofuels are being developed as a green and sustainable alternative to fossil fuels for transportation, heating and energy supply. Rising oil prices have made the production of biofuels more economically feasible, and ultimately the availability of fossil fuels is limited. Bioethanol, a biofuel, is generally considered as being much more CO2 neutral in comparison with petroleum based transportation fuel. In addition, it is possible to use bioethanol as a partial as well as full gasoline substitute without drastic changes to the engine technology.
Typically, bioethanol is produced by fermentation of sugars derived from agricultural feedstock—such as sugar cane, sugar beet, maize and cereals (such as wheat and corn)—which are starch—rich and sugar-rich plant materials (the remainder of these plant materials is referred to as agricultural waste). However, a problem associated with processes which use these materials is that they utilise what could otherwise have been used for foodstuffs for humans and animal feeds. A consequence of this is that there is a reduction in the amount of foodstuffs and animal feeds which are available which, in turn, increases the price of food.
In fact, it has been predicted that even if the entire maize crop of the USA was used for ethanol production it would not be possible to meet the future demand in the USA. For example, in Spring 2008 the US Department for Agriculture estimated that, based on the amount of US land sown with maize, about 12 billion bushels of maize would be harvested in USA in 2008. Using techniques currently available in the art, 2.8 gallons of ethanol is the average production from 1 bushel of maize. Thus, if the entire 2008 USA maize harvest was made into ethanol, 33 billion gallons of ethanol would be obtained. According to the US Energy Information Administration statistics, however, 142 billion gallons of gasoline was used as fuel for cars and trucks in 2007 in USA. Assuming that there is no significant decline in the demand for gasoline in the USA in 2008, then the supply of the gasoline substitute ethanol from maize could not meet the demand.
Thus, the availability of sources of starch-rich and sugar-rich plant materials is a rate-limiting factor for the production of biofuels.
The so-called agricultural waste material of, for example, sugar cane, sugar beet, sorghum, Soya beans, maize, and cereals (such as wheat and corn) comprises mainly lignocellulosic material. Lignocellulosic material primarily comprises long sugar chains. In general, about two thirds of the sugars of these long chain sugars are hexose sugars (in particular glucose), which are mainly in the form of cellulose, and about one third of the sugars of these long chain sugars are pentose sugars (in particular xylose and arabinose) present mainly in the form of arabinoxylan polymers. After hydrolysis of cellulose, the hexose sugars can be fermented by the traditional yeast based method. However, cellulose is a robust structure which is very resistant to extraction and enzymatic hydrolysis. Arabinoxylans are comparatively easier to extract and hydrolyse to release, in the main, pentose sugars; but the released sugars can not be fermented into ethanol in sufficiently high concentration by known microorganisms.
Two obstacles for efficient ethanol production from waste plant material are the difficulties associated with depolymerisation of cellulose and the lack of suitable organisms that can metabolize, for large-scale production, pentose sugars into ethanol.
Theoretically, one way to obtain such a suitable organism is to transfer the ability to metabolize pentoses from natural pentose metabolizing organisms into known highly efficient ethanol producers. This has been the subject of much work in various research groups during the last 15-20 years. But with pentoses it has not been possible to obtain metabolic rates comparable to the rates obtained when using glucose. To increase this low metabolic rate has been and is still a subject of discussion and ongoing research and is still a major obstacle for the use of, for example, engineered S. cerevisiae in ethanol fermentation from pentoses.